Field of the Invention
The present invention relates to a heat exchanger for arrangement in a service liquid tank, in particular in a motor vehicle tank, comprising:                a heat-exchanger liquid reservoir for accepting a supply of liquid,        an electric heating device that is constructed and disposed to transfer heat into the heat-exchanger liquid reservoir, and        a heat-exchanger line that originates at least from the heat-exchanger liquid reservoir, and which is constructed for the transfer of heat from the liquid flowing in the heat-exchanger line to an area outside the heat-exchanger line.        
Description of the Related Art
A heat exchanger of the above-described type is known from DE 10 2009 028 113 A1.
It is known from this publication to heat liquid with an electric heating device inside a heat-exchanger liquid reservoir separate from the reservoir volume of the motor vehicle service tank, and pump it away from the heat-exchanger liquid reservoir via a line. More specifically, in the case of the known heat exchanger the liquid is pumped from the heat-exchanger liquid reservoir out of the motor vehicle service-liquid tank for its intended use. Here the line of the known heat exchanger extends through the reservoir volume of the motor-vehicle service-liquid tank and then exits the motor-vehicle service-liquid tank, wherein when flowing through the line, which is actually not a primary heat-exchanger line but rather a pumping line, whose pumped material unavoidably gives off some heat during the pumping; due to the short length of time the heat-exchanger liquid remains in the line, only a small amount of heat from said liquid can be transferred to the reservoir volume of the service-liquid tank.
A similar service-liquid tank with heat exchanger is also known from DE 10 2009 029 313 A1.
The service-liquid tank in the last-mentioned publication has an additional heat-exchanger line. The heat-exchanger line, through which either cooling liquid or exhaust gas from the internal combustion engine of a motor vehicle flows, passes through the reservoir volume of the service-liquid tank in order to emit heat from the heat-exchanger fluid: exhaust gas or coolant liquid, to the reservoir volume of the service-liquid tank. The exhaust gas or the coolant liquid is not heated by an electric heating device, because the internal combustion engine serves as the heat source.
The electric heating device is additionally present in the service-liquid tank, so that it can, as in the publication covering the device of the above-described type, also itself melt service liquid locally in the tank, when the internal combustion engine is still cold, and the coolant liquid is therefore not temperate enough to heat the service liquid, or an exhaust gas might not yet be flowing, because the internal combustion engine has not yet been put into operation.
In the case of the heat exchanger of the above-described type, the service liquid stored in the actual service-liquid tank is used as the heat-exchanger liquid.
In DE 10 2009 029 313 A two different heat-exchanger liquids are used, in one instance, as in the publication covering the device of the above-described type, it is the actual service liquid stored in the tank, and in the second instance, it is the fluid explained above, in the form of either exhaust gas or of coolant liquid of the internal combustion engine.